Radiochemical Yields for Cross-Links and Branches in Radiation-Modified Poly(tetrafluoroethylene)

For the enhancement of the compatibility of fluoropolymers with other materials radiation modification is applied, for which irradiation with high-energy electrons provides an efficient and flexible possibility. Poly(tetrafluoroethylene) (PTFE) is known to predominantly undergo chain scission under irradiation. To counteract this chain scission and the resulting deterioration of the mechanical properties, PTFE has been irradiated at temperatures above the melting temperature under inert atmospheres. Under these conditions, in addition to chain scission long chain branches and cross-links are formed. The resulting structures have been studied by high-resolution fluorine-19 solid-state NMR applying high-speed magic-angle sample spinning. The structural changes as a function of energy dose and irradiation condition have been investigated upon irradiation under nitrogen atmosphere at 385 °C and under vacuum at 365 and 385 °C. Structures of interest are terminal groups, CF3 side groups, branches, and cross-links. From quantitative information derived from the 19F NMR spectra, the radiochemical yields, G(x) values, for the formation of CF3 side groups, branches, and cross-links have been calculated. G(x) values of side groups and branches decrease with radiation dose, while the radiochemical yield of cross-links is nearly constant. These results are interpreted that in the first step in addition to main-chain scission the CF3 side groups and branches were formed and in the second step after forming a sufficient quantity of side groups and branches the cross-links are formed.